Printing technology has evolved over the centuries from a discipline considered an art form to one now dominated by the strictures of organic and surface chemistry. In the present context, inks are film forming compositions that dry to a thickness between 0.2 and 30 microns. In general, inks consist of two major components: the colorant (a pigment or dyestuff) and the vehicle (the liquid carrier that suspends or dissolves the colorant). The primary functions of the vehicle is to carry the colorant to the printed surface and thereafter harden and bind the colorant to the surface.
Although the technology underlying ink printing extends back several centuries, the industry supports a high profile, fast paced rate of technical advancement. The contours of the marketplace are continually placing new restrictions and pressures on ink developers. Recently, this is reflected by the advanced computer control presses capable of speeding the stock through the various printing stations at unprecedented rates. Although this permits substantial increases in printing output, it significantly reduces the attendant drying time. This translates to ink formulations having a requisite dry/cure period that corresponds to the faster cycles.
A related development involves worker and the general public's exposure to specific hazardous compounds in the plant environments and nearby community. Occupational Safety and Health Agency (OSHA) and Environmental Protection Agency (EPA) requirements regarding toxic and potentially carcinogenic materials have significantly restricted the permitted environmental concentrations of commercially important solvents. In response, the solvent loading and heavy metal drying agents in ink formulations must be handled in a manner that inhibits the build-up of solvents and/or other toxins in the plant, its environment and its waste streams.
It can be recognized that the above factors have created a strong incentive to printers to reduce and/or eliminate the solvents and heavy metal dryers employed in their ink formulations. The problem, of course, is that ink performance is usually tied directly to threshold levels of solvents that preserve low viscosity, spreadability, and good color (pigment) distribution. There has, heretofore, been a trade-off between speed and performance, and the above-noted environmental concerns.
Prior art ink systems have normally consisted of organic composition comprising oils and resins, appropriate viscosity controlling solvents, dryers and of course the requisite pigment or dyestuff. During the printing operation, the composition is selectively applied to the substrate and thereafter cured. Exemplary constituents include drying vegetable oils such as glyceride esters of unsaturated fatty acid esters. The unsaturation or double bond content of these esters permits a spontaneous cross-linking reaction in the presence of oxygen. For example, linolenic acid (the main constituent of linseed oil) and its isomeric varieties have long been effective drying components of coating and printing ink compositions. Attention is directed to The Printing Ink Manual, Van Nostrand Reinhold (Int'l) Co., Ltd. 4th Ed. (1988) which is herein incorporated by reference as if restated in full.
Oils have often been combined with selected resins such as (polydicycyclo) pentadiene, rosin, polyterpenes and alkyds to promote drying and film integrity over time. Both natural and synthetic resins have been used, but the major difficulty in application remains the matching of oil/resin properties that address a broad level of requirements. There continues to exist a market need for new and improved formulations of coatings and inks such that the liquid vehicles are substantially solvent-free and also suitable for simple, quick and low cost effective curing, without the need for additives, e.g., hazardous organic peroxides or radiation for radical formation or heavy metal ions as so-called driers, e.g., cobalt and manganese.
It was the above challenges that gave rise to the development of the present invention.